fix: Enforce one-terminal-per-Request across respond() calls

Author: thced

src/main/java/com/retailsvc/http/Request.java

@@ -16,6 +16,7 @@ public final class Request {
   private final String operationId;
   private final Map<String, String> pathParameters;
   private final Map<String, TypeMapper> bodyMappers;
+  private boolean responseSent;
 
   public Request(
       HttpExchange exchange,
@@ -53,6 +54,9 @@ public String header(String name) {
   }
 
   public ResponseBuilder respond(int status) {
-    return new DefaultResponseBuilder(exchange, status, bodyMappers);
+    if (responseSent) {
+      throw new IllegalStateException("Response already sent");
+    }
+    return new DefaultResponseBuilder(exchange, status, bodyMappers, () -> responseSent = true);
   }
 }

src/main/java/com/retailsvc/http/internal/DefaultResponseBuilder.java

@@ -17,14 +17,16 @@ public final class DefaultResponseBuilder implements ResponseBuilder {
   private final HttpExchange exchange;
   private final int status;
   private final Map<String, TypeMapper> mappers;
+  private final Runnable onTerminated;
   private final Map<String, String> pendingHeaders = new LinkedHashMap<>();
   private boolean terminated;
 
   public DefaultResponseBuilder(
-      HttpExchange exchange, int status, Map<String, TypeMapper> mappers) {
+      HttpExchange exchange, int status, Map<String, TypeMapper> mappers, Runnable onTerminated) {
     this.exchange = exchange;
     this.status = status;
     this.mappers = mappers;
+    this.onTerminated = onTerminated;
   }
 
   @Override
@@ -125,6 +127,7 @@ public void close() throws IOException {
   private void terminate() {
     checkNotTerminated();
     terminated = true;
+    onTerminated.run();
   }
 
   private void checkNotTerminated() {

src/test/java/com/retailsvc/http/RequestTest.java

@@ -1,9 +1,12 @@
 package com.retailsvc.http;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 import com.retailsvc.http.internal.DispatchHandler;
+import com.sun.net.httpserver.Headers;
 import com.sun.net.httpserver.HttpExchange;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicReference;
@@ -44,4 +47,17 @@ void readsBoundContext() throws Exception {
     assertThat(seenOpId.get()).isEqualTo("get-x");
     assertThat(seenPathParams.get()).containsEntry("id", "42");
   }
+
+  @Test
+  void respondAfterTerminalThrows() throws Exception {
+    HttpExchange exchange = mock(HttpExchange.class);
+    when(exchange.getResponseHeaders()).thenReturn(new Headers());
+    Request req = new Request(exchange, new byte[0], null, "op", Map.of(), Map.of());
+
+    req.respond(204).empty();
+
+    assertThatThrownBy(() -> req.respond(200))
+        .isInstanceOf(IllegalStateException.class)
+        .hasMessageContaining("already sent");
+  }
 }